Improve combobox single item styling

[timroes]

We currently render a singleSelection={true} combobox the same as any other:

I think we should rethink that behavior, since this combobox still looks a lot like you would be able to select multiple values, since the single value is rendered in it's tag form. To be a proper replacement to the current aggregation selection we would need the selected value to look like a single value to not misguide the user.

[cchaos]

@timroes To better understand your use case, why would you decide to use the EuiComboBox over a EuiSelect for single selection?

[timroes]

Because we want grouping and search available in the component.

[snide]

Labeling this style only since it doesn't involve a functionality change. I agree we could probably improve the styling a bit, but I wouldn't consider this a blocker at the moment.

[cqliu1]

This is similar to my request in #854.

[cchaos]

This was fixed in #1139 via singleSelection.asPlainText: bool